When an x-ray or a γ-ray penetrates a substance, absorption and scattering differ depending on a type and a shape of a constituent substance. Recording the above as a picture such as a photo, a video, and a digital file enables obtaining a broken state, change, a packing condition or the like of the substance. This method is generally used as a method for examining a state of the inside of a human body by an x-ray picture. The above method for measuring a state of the inside without destructing a substance or a sample which is desired to be measured is called radiography or a nondestructive radiation imaging method.
In x-ray photographing used for medical diagnosis or industrial nondestructive inspection, usually an x-ray film is combinedly used with a radiation intensifying screen in order to improve sensitivity of photographing system. In x-ray photographing, silver particles on the film are directly blackened by an x-ray transmitting a sample, and besides, the x-ray is converted into a visible light in the intensifying screen and the light blackens silver particles on the x-ray film, whereby a transmission image of the sample is obtained.
There is a method in which a line sensor is used as a sensor and a subject is scanned by the line sensor when the subject passes, so that a transmission image is measured nondestructively, as in baggage inspection at an airport. In such nondestructive inspection methods using an x-ray, in a case of a composite material or in a case that thicknesses of samples to be transmitted are quite different, a transmitted radiation dosage becomes extremely large or small depending on parts, so that a larger radiation dosage flows to a smaller radiation dosage part, causing a phenomenon called fog or halation and making a result of photographing hard to view. As a method for solving such a phenomenon, there are suggested methods in which a sensitivity region of a sensor is enlarged by color information and so on.
In an atomic power plant or the like, radioactive waste is filled and stored in a sealed vessel such as a drum and a container. A stored amount thereof is increasing recently and volume reduction is required. Since a processing method for volume reduction differs depending on a material, it is necessary to sort contents by material, but there is a risk in a sorting operation. When the radioactive waste and the drum as a whole undergo a melting process, for example, a plasma melting process, an operator opens the drum and takes out the radioactive waste in the drum to sort. Since aluminum and lead in particular are not suitable for the melting process, it is necessary that aluminum and lead are stored without undergoing the melting process. It is because if aluminum undergoes the melting process and is solidified with concrete, there is a possibility that moisture in the concrete and aluminum react to generate hydrogen and damages a melting furnace. Further, if lead is molten, toxic gas is generated unpreferably in terms of pollution prevention. Therefore, when the melting process in particular is performed, a method and a device which can easily judge that aluminum or lead does not exist in the drum nondestructively are desired.
As an example of a conventional sorting process of waste as described above, there is a method in which whether or not a metal exists in the waste is sorted by a metal detector and further whether a shape of the metal is indefinite or definite is sorted by an x-ray inspection device (for example, see Reference 1). In addition, there is a method in which a component of radioactive waste moving on a roller conveyer is found out by combination of a capture γ-ray analyzing device and a fluorescent x-ray analyzing device (for example, see Reference 2). The former technique of the above conventional techniques cannot identify types of the metals, while the latter technique can identify a material but opening is required. On the other hand, as a method for judging materials of contents nondestructively, there is an x-ray CT method. In the x-ray CT method, an x-ray absorption rate, that is, an x-ray absorption coefficient of each waste is obtained and a thickness of each waste is obtained, whereby the material of each waste can be judged. However, in this method, a device constitution becomes complicated for the sake of data collection, image reconstruction computation, displaying, or the like, thus leading to a high cost.
As a method for solving the above, there is a following method. In this method, by using an n x-ray tube voltage as a parameter, an x-ray transmission image tone value or a function thereof is detected to be a reference tone for every material. The reference tone can be also obtained by calculation and is registered as a data table. A sample is measured and compared with the reference tone registered as a database in advance, whereby a material is specified. In this method, identification of lead and aluminum in particular can be performed more easily than in an x-ray CT (for example, see Reference 3). However, in this method, there are problems that when different materials overlap with each other in a perspective direction, it is difficult to judge from a reference function for every material, and when a thickness is different from that in the reference function, judgment is also difficult, and so on. Further, in this method, by using thickness dependence of x-ray transmission by material by an x-ray tube voltage (same as energy, but strictly, not equal because it has a distribution) as a parameter, judgment is done based on an obtained luminance value. However, in a case that the tube voltage is used as the parameter, discriminable luminance data cannot be obtained unless a tube current is simultaneously used as a parameter. For example, in a case of a tube current of data of 300 kV and a tube current of data of 100 kV, similar luminance cannot be obtained unless the tube current of 100 kV is increased. Therefore, it is necessary to have large data for every material as a table, and in order to perform accurate judgment, a measurement condition of a sample must be the same as a condition under which a database is obtained. However, in a case of waste, materials and sizes of contents vary, and photographing cannot be always done under the same condition. Therefore, there is a problem that the database is required to be remeasured every time the measuring condition is altered.
Reference 1: JP-A 6-273588 (KOKAI)
Reference 2: JP-A 7-209493 (KOKAI)
Reference 3: Japanese Patent JP-B2 3193665